Make filegroup paths relative to module dir in mixed builds

filegroup.srcs must have a .rel() version of their paths
that is relative to the module directory. Previously in
mixed builds, the relative path wasn't filled in.

Bug: 229251008
Bug: 204244290
Test: go tests, and USE_BAZEL_ANALYSIS=1 m apexer with the following cl
Change-Id: I11a2a8b96139e0e7b956fa4c256a9a99c02f88f3
diff --git a/android/filegroup.go b/android/filegroup.go
index 14ed783..9e5769a 100644
--- a/android/filegroup.go
+++ b/android/filegroup.go
@@ -178,8 +178,7 @@
 
 	bazelOuts := make(Paths, 0, len(filePaths))
 	for _, p := range filePaths {
-		src := PathForBazelOut(ctx, p)
-		bazelOuts = append(bazelOuts, src)
+		bazelOuts = append(bazelOuts, PathForBazelOutRelative(ctx, ctx.ModuleDir(), p))
 	}
 
 	fg.srcs = bazelOuts